When to use add shader?
I think Maccesch meant that in the context of reflecting shaders.

Think of it this way:


In a diffuse+translucent material, you want the light transmitted through in addition to the light reflected off the surface.

Though you may want to darken the translucent node a bit to simulate absorption:


In a diffuse+glossy material, you want some of the light to be reflected specularly (glossy), and some to be reflected and scattered (diffuse).

Here "some"  can be conveniently defined by the factor of a mix node (which in this case is defined by the angle of the surface to the camera, via a fresnel node). Using this setup, at most you can reflect the same amount of light as received:



You don't want to reflect more light than what hits the surface, as easily happens with an add shader:




The exact difference between the two nodes is nicely covered in question you linked.